1. Technical Field of the Invention
The present invention relates generally to an optical information recording and/or erasing system, and more particularly to an optical information recording and/or erasing system with a servo control circuit for adjustment of focusing and tracking errors of an optical system relative to an information recording medium such as an optical disk.
2. Background Art
The present invention represents an improvement on an optical information recording/reproducing system, as taught in Japanese Patent First Publication No. 3-141039. This prior art system has a servo control circuit for feedback-controlling an optical system to compensate for a focusing error during record and reproduce modes of operation. When a semiconductor laser is employed as a light source for recording and reproducing information data on and from an optical disk, a change in power of the light source between the record and reproduce modes of operation will cause the wavelength of light from the light source to be changed, leading to variation in refractive index of an objective lens of the optical system. This results in an optical spot formed by the objective lens of the optical system on the optical disk remaining defocused until a focus tracking operation has been completed. For avoiding this drawback, the prior art system provides the offset to a servo control signal to compensate for the misalignment of the optical spot due to the variation in wavelength of the light from the light source.
The above prior art system, however, experiences the following focusing difficulties during the recording. The objective lens takes about 100 .mu.s to 1 ms to reach a record focusing position where the optical spot is exactly focused on the optical disk after the offset is applied to the servo control signal. Therefore, when a desired address recorded on the optical disk is detected immediately after the application of the offset (e.g. after 10 .mu.s), an information data-recording operation is undesirably initiated before the objective lens has reached the record focusing position completely, thereby causing a leading portion of the information data to fail to be recorded.